1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method, particularly to an ink jet printing apparatus and an ink jet printing method ejecting black ink and color ink.
2. Description of the Related Art
In an ink jet type printing apparatus, it is keenly demanded for increasing the printing speed and image quality in color printing using a plain paper or the like. The following techniques are generally known as techniques to increase the printing speed. That is, in addition to increasing the length of a printing head; generally, the printing (drive) frequency of the printing head is increased; and bidirectional printing is carried out. Particularly, compared to one-way printing, the bidirectional printing decentralizes the energy necessary for obtaining the same throughput on the time basis. Therefore, the bidirectional printing is an effective method as a total system for reducing the cost.
However, the bidirectional printing has a problem as described below. That is, depending on the arrangement structure of a printing head, the ejecting order (application order) of color inks differs between a forward direction and a backward direction of the main scanning. This may cause a stripe-like unevenness of colors. As an arrangement for solving such unevenness of color caused by the difference of ink application order, there is known a printing head in which the ink nozzle arrays are symmetrically arranged (for example, Japanese Patent Laid-Open No. 2001-171119).
When printing a text or the like mainly on a plain paper, an ink jet printing apparatus which uses pigment type ink to achieve high quality in black characters becomes a major trend. In many cases, pigment type black ink is prepared as a low permeable composition to prevent deterioration in quality due to feathering. When a printing head with the symmetrically arranged nozzles being disposed parallel to nozzles which eject black ink of low permeation property in a main scanning direction is used, the symmetrically arranged nozzles ejecting a color ink of high permeation property the permeability of which is relatively high, image degradation may be caused. That is, exudation (bleed) or white blur phenomenon (decrease of density due to sinking of ink of low permeation property) may occur in a boundary between black and color. Therefore, in many cases, a printing head, in which a nozzle array ejecting black ink and a nozzle array ejecting color ink are disposed parallel to each other, is employed. To be more precisely, there is used a printing head having a nozzle arrangement in which a nozzle array of black ink is longer than a nozzle array of color ink, and a part of area printed by the nozzle array of black ink overlaps with a area printed by the nozzle array of color ink.
However, when gray is printed using the above-described printing head having a vertical arrangement, unevenness of color may occur. For example, when gray color is printed in a bidirectional printing by mixing dots of black ink and dots of color ink, in each of the scan areas (bands), eject timing of the black ink and the color ink is different from each other in a right end portion and a left end portion of the respective scan areas. That is, in scanning of the printing head, the scan areas corresponding to the nozzles ejecting black ink and the nozzles ejecting color ink are different from each other. Therefore, in a predetermined scan area, the nozzles ejecting black ink perform a former scan (first scan) and the nozzles ejecting color ink perform a latter scan (second scan). Alternatively, the nozzles ejecting color ink perform the first scan and the nozzles ejecting black ink perform the latter second scan. Therefore, the tone of the same gray may differ from each other in the end portions of the two scan areas. As a result, a stripe-like unevenness of color may be generated in the bands on the entire printing medium.
FIGS. 10A and 10B illustrate density differences which occur in a specific area when the printing is carried out using a printing head having a vertical arrangement. Referring to FIG. 10A, in a first main scanning (first forward scan), in the uppermost band (first area), printing is carried out only in an area of black ink nozzle array which is located not in adjacent to the color ink nozzle array. After completing the first main scanning, the sheet is fed by a distance equivalent to one band width. Subsequently, the second main scanning, which is the scanning in the backward direction, is carried out. In the second main scanning (second backward scan), the printing with color ink is carried out corresponding to the color ink nozzle array in the uppermost band (first area) where the black ink has been ejected. Further, in the next band (second area), printing with black ink is carried out corresponding to an area of the black ink nozzle array which is located not adjacent to the color ink nozzle array. In this case, an end area B in the uppermost band (first area) is an area where the ink of respective colors is ejected before and after the scanning direction of a print head is switched. On the other hand, in an end area “A”, the ink of respective colors is ejected with a time lag equivalent to one reciprocation movement.
When such bidirectional printing is repeated, particularly in areas adjacent to the ends of the respective bands, applied times of the black ink and the color ink are largely different from each other. In FIG. 10A, end areas A, C and E in the respective bands are the areas where a difference between the applied times of the black ink and the color ink becomes maximum. On the other hand, end areas B, D and F are the areas where the time difference becomes minimum. In the end areas other than the above also, the black ink and the color ink are applied with a time difference corresponding to the respective positions.
Therefore, when the printing is carried out using, for example, black ink of low permeation property and color ink of high permeation property, in the other end of a printing medium, there may be such a case that the color ink is applied before the black ink is applied and permeates into a printing medium. Therefore, the black ink is pressed into the printing medium resulting in a reduction of the density. That is, even when the printing is carried out using the same application amount and in the same application order, in the right end portions and the left end portions of the printing medium, density difference may occur due to the ejecting time difference of the ink.
That is, in the areas A, C and E where the ejecting time between the black ink and the color ink is a longer time, the color ink is applied in a state that the black ink is fixed to some extent as shown in FIG. 10B. Therefore, high density gray is printed. On the other hand, in the areas B, D and F, the ejecting time between the black ink and the color ink is a shorter time as shown in FIG. 10B. Thus, depending on the ejected time difference between the black ink and the color ink, a density difference as “dark” or “light” is repeated alternately in the unit of band width, particularly in the end areas of the bands.
The magnitude of density difference due to the time difference in one band changes stepwise in a scanning direction of the printing head. Therefore, the density difference is hardly recognized visually. However, in the case of density difference which occurs continuously over several bands, in the right and left end areas, the density difference due to the difference of printing time is alternately generated. Therefore, the density differences among bands become conspicuous, resulting in deterioration of image quality.
As a technique to prevent such image degradation due to density difference, there is known a technique in which when the density difference occurs due to the printing time difference, a certain period of waiting time is provided between the printing with the color ink and the printing with the black ink (for example, Japanese Patent Laid-Open No. 2004-181698). According to the above technique, the waiting time allows the previously ejected ink to permeate into a printing medium before the next ink is ejected. The density difference due to printing time difference is reduced and the image degradation can be suppressed.
However, according to the above technique, when the printing, in which the printing time lag causes the density difference, is carried out, the printing is restricted and waited until the ink permeates into the printing medium. As a result, the printing speed is reduced.